This invention relates to an improved coating for a gypsum-based floor underlayment and a method of preparing it. More specifically, the coating provides a harder surface for the floor underlayment and promotes better adhesion of the coating to the underlayment surface.
Gypsum-based building materials are useful in the building industry in a number of applications. The materials are prepared by mixing a slurry made of calcium sulfate hemihydrate (also known as calcined gypsum, plaster of paris or calcium sulfate semihydrate) in water and pouring it into a mold, forming a panel or floor underlayment, or making the slurry into any appropriate shape. Water reacts with the hemihydrate to form an interlocking matrix of calcium sulfate dihydrate (also known as gypsum) crystals, causing the slurry to set and harden. Ideally, when an aggregate is present in the slurry, it is evenly dispersed through the matrix, adding strength and hardness to the resulting matrix layer. Excess water is allowed to evaporate or is driven off by drying the gypsum product. Gypsum is less expensive, lighter weight and faster setting than concrete. The sound reduction and fire retarding properties of gypsum materials are superior to those of wood or other cellulosic building materials.
When used as a floor underlayment, gypsum-containing compositions are coated with a sealant. A mastic or adhesive is then applied to the sealant, and flooring is applied to the mastic. The flooring materials include vinyl, linoleum, and other floor coverings that are glued directly to the underlayment.
However, gypsum-based floor underlayments are not universally acceptable due to soft surface characteristics and the presence of powder or dust at or near the surface of the underlayment. Under some circumstances, the dihydrate matrix layer with the aggregate substantially dispersed therein, is topped by a powdery layer that forms on top of the matrix layer. At the surface, the dust is unsightly and inconvenient to remove. When located just below a surface crust, small amounts of wear cause the surface crust to break, exposing the powdery layer. Even if time and expense is invested in removal of the powdery layer, the resulting underlayment has an uneven surface.
Formation of the powdery layer has been observed when extreme retarders, such as polycarboxylates, are used in the underlayment. Set retarders are frequently used in the slurry composition to lengthen the time that the slurry is workable, known as the open time. Increasing the open time provides the installers additional time to pour the floor, level it and screed it before the extent of the setting reactions prevent working of the surface. In this case, analysis shows the powdery layer is primarily a mixture of unreacted calcium sulfate hemihydrate and calcium sulfate dihydrate.
Polycarboxylates are known to set retarders but also improve the flowability of the calcium sulfate hemihydrate slurry, providing a pourable and/or self-leveling composition using less water. Decreasing the amount of water in the slurry increases the strength of the product and reduces drying time. The benefits of using polycarboxylates in flooring underlayments are disclosed in co-pending Patent Cooperation Treaty PCT/US03/09400, filed Mar. 27, 2003, designating the United States of America and entitled, “High Strength Flooring Compositions,” herein incorporated by reference in its entirety.
However, the problem of unreacted calcined gypsum is not limited to cases when retarders are used. Environmental conditions, such as wind, result in increased surface drying. Overwatering of the slurry allows separation of the aggregate and the gypsum matrix before the setting reactions are complete. Yet another example of a situation that results in unreacted calcium sulfate hemihydrate is where there is a change in raw materials. If the aggregate is changed from a wet sand to a dry sand, the effective amount of water in the slurry is reduced and in some cases is insufficient to react all of the hemihydrate. Thus, the problem of finding unreacted calcium sulfate hemihydrate at the surface of a gypsum-based floor underlayment is not uncommon and can occur in a variety of situations.
Conventional methods of treating underlayment surfaces fail to cure the problem of unreacted hemihydrate. Merely spraying water on the powdery layer is not sufficient to hydrate a significant portion of the unreacted hemihydrate. Application of conventional topcoating materials, such as TOPSEAL (United States Gypsum Company, Chicago, Ill.), temporarily seals the dust between the coating and an interlocking matrix of calcium dihydrate crystals, but does not provide a long-term solution to this problem. When the powdery layer forms under the surface crust, the coating layer adheres to the crust as it dries, but the coating and attached crust easily peal away from the underlying matrix layer because the powdery layer does not attach the crust to the strong matrix layer. In places where the powdery layer is thin, the coating can penetrate through the powdery layer to bond to the dihydrate matrix structure in isolated places. However, isolated bonds may not be strong enough to hold the coating in place over the entire surface. As the coating over the unbonded areas pulls and shifts under normal use, the bonded areas are pushed and pulled in many directions by movement of the coating. Changing pressures can cause the small patches of bonded gypsum to break up, eventually causing delamination of all or of large portions of the coating.
Another method of combating this problem would be through the use of additives to the calcined gypsum slurry. However, this technique requires that one accurately guess in advance the amount of the powdery layer that will appear as the underlayment sets. Variables such as wetness of the aggregate, wind conditions, temperature, type and amounts of set retarders used would have to be considered, resulting in a complex algorithm for determining if such additives would be necessary. If the amount of unreacted calcium sulfate dihydrate differed from the predicted amount, or if conditions changed in the middle of the job, the amount of the additive would have to be reconsidered, requiring reevaluating all of the above variables. This could lead to a lengthy and complex process that installers will not want to use.
Thus, there is a need in the art for a coating to strengthen the surface of a floor underlayment, especially when the underlayment has a powdery layer at or near its surface. The coating should also react with unreacted calcium sulfate hemihydrate to improve the strength of the powdery layer. The coating should be easy to apply using commonly available tools or methods.